The Fragile Families and Child Wellbeing Study is a longitudinal birth cohort study of approximately 3700 unmarried parents and 1200 married parents and their children. We seek funding to: 1) perform assessments at age 9 of children's health, language ability, academic achievement, behavior problems and social relationships;2) conduct interviews with children's biological parents, to update information on family resources and parental relationships;3) collect saliva samples for genotyping from primary caregivers and children;4) Interview social parents who live with the child, and;5) collect information from children's teachers on child outcomes and school characteristics. These data will allow researchers to answer the following questions: How do children develop over time, and how do family resources influence children's health and development? How do the resources of unmarried parents evolve over time, relative to those of married parents? How do children's genetic endowments interact with their environments to influence their outcomes? What role do social (non- biological) parents play in the lives of children? How do school environments influence children's social and academic outcomes? The study includes a large number of children and adults who are: poor, members of racial and ethnic minority groups, immigrants and disproportionately affected by welfare, child support, child care, and incarceration policies. The children have diverse and often unstable family structures and child care arrangements. Thus, the sample is well-suited to study how multiple sources of disadvantage influence child health and development. The data are also unique in containing detailed information on mothers, fathers, and their relationships with each other. The information on unmarried fathers, in particular, allows us to follow a group of men that is often under-sampled in national surveys.